1. Field of the Invention
The invention pertains to a hydrodynamic clutch device of the type having a pump wheel; a housing connecting the pump wheel to a drive; a turbine wheel connected to a takeoff, the turbine wheel being located in the housing and cooperating with the pump wheel to form a hydrodynamic circuit; and a bridging clutch located in the housing and having a piston located between the hydrodynamic circuit and a pressure space, the piston being movable between an engaged position, wherein the drive transmits torque to the takeoff via the bridging clutch, and a released position, wherein the drive transmits torque to the takeoff via the hydrodynamic circuit.
2. Description of the Related Art
U.S. Pat. No. 5,575,363 describes a hydrodynamic clutch device designed as a hydrodynamic torque converter. This device comprises a clutch housing, which is brought into connection in the conventional manner for rotation in common with a drive, such as an internal combustion engine, and a pump wheel, which works together with a turbine wheel and a stator to form a hydrodynamic circuit. Whereas the turbine wheel is connected nonrotatably to the takeoff, such as a gearbox input shaft, the stator is mounted by way of a freewheel on a support shaft, which is provided radially between a pump wheel hub and the gearbox input shaft. In addition, the hydrodynamic clutch device has a bridging clutch with a piston, which is connected nonrotatably but with freedom of axial movement to the clutch housing.
The hydrodynamic clutch device is designed as a two-line system, as a result of which the following pressure and flow conditions are produced:
A first pressure-medium line is connected to a first flow route, which has flow channels radially between the pump wheel hub and the support shaft and additional flow channels radially between the support shaft and the gearbox input shaft. This first pressure-medium line is formed by flow channels provided in the thrust washers located on both sides of the freewheel of the stator. Clutch fluid is supplied to the hydrodynamic circuit through these channels. When there is a positive pressure in the hydrodynamic circuit, the piston is pushed toward the adjacent housing cover of the clutch housing; friction surfaces then allow the piston to be carried along rotationally by the clutch housing. Conversely, this rotation in common produced by the friction surfaces is released when, through a second pressure-medium line, a pressure space assigned to the piston and located axially between the piston and the housing cover is supplied with a positive pressure versus the hydrodynamic circuit, as a result of which the piston is pushed axially toward the hydrodynamic circuit. The second pressure-medium line is connected to a second flow route, which passes by way of a center bore in the gearbox input shaft. Each of the two flow routes is connected to a fluid reservoir.
The essential principle of a two-line system of this type—but also its essential disadvantage—is the presence of the bridging clutch as a separation point between the hydrodynamic circuit and the pressure space. When the bridging clutch is open, therefore, a connection exists between the hydrodynamic circuit and the pressure space, which allows the pressure to equalize at least in the area of the radial extension of the bridging clutch, whereas, when the bridging clutch is closed, a pressure which can differ considerably from that in the pressure space can easily build up in the hydrodynamic circuit, even in direct proximity to the bridging clutch. This situation is not changed even if grooves are provided in the bridging clutch, because, measured against the total amount of clutch fluid supplied to the hydrodynamic circuit and the pressure space, such grooving never allows more than a very small leakage flow to pass through and is thus unable to exercise any noticeable effect on the pressure conditions in the two pressure spaces.
Especially during operation in push mode, that is, when the takeoff rpm's are higher than the drive rpm's, this situation has disadvantageous effects as soon as the bridging clutch is to be closed for the purpose of taking advantage of the braking action of the drive to reduce or avoid a long period of efficiency-impairing slippage or to prevent an unbraked acceleration of the drive upon a sudden transition from push mode to pull mode. The following unpleasant effect then occurs:
As a result of the filling of the hydrodynamic clutch device with clutch fluid, this fluid pushes its way radially outward under the effect of centrifugal force, and ideally we can assume a pressure of “zero” at the center of rotation of the clutch device. As the distance from the center of rotation increases, however, the pressure values increase monotonically, near-maximum values being reached in the area of the radial extension of the bridging clutch, which is usually located in the radially outer area of the device. The increase in these pressure values during operation in push mode is more pronounced in the hydrodynamic circuit than in the pressure space, because the clutch fluid in the pressure space rotates essentially at the same speed as the clutch housing, whereas in the hydrodynamic circuit it rotates at the higher takeoff-side speed of the turbine wheel. Under consideration of the boundary condition that, when the bridging clutch is open, the pressure conditions within the area of the radial extension of the bridging clutch are equalized between the hydrodynamic circuit and in the pressure space, the difference between the pressure-increase curves on the two sides of the piston have the effect that the course of the pressure increase in the pressure space—starting from the area of the radial extension of the bridging clutch and leading radially inward from there—undergoes less of a pressure drop than the course of the pressure increase on the opposite side of the piston, that is, in the hydrodynamic circuit. The consequence of this is that the pressure in the part of the pressure space radially inside the bridging clutch is higher than that in the hydrodynamic circuit, as a result of which the piston is held stably in the released position. If, under these conditions, an actuating command is given to close the bridging clutch, a positive pressure must first be built up in the hydrodynamic circuit which significantly exceeds the pressure in the pressure space. There is a therefore a considerable delay in the closing of the bridging clutch.
As soon as the piston of the bridging clutch starts moving toward its engaged position after the necessary high positive pressure has been built up in the hydrodynamic circuit, the connection between the hydrodynamic circuit and the pressure space becomes smaller and thus acts increasingly as a throttle, which has the effect of lowering the pressure in the pressure space below that present in the hydrodynamic circuit and thus ultimately causes the sign of the axial force acting on the piston to reverse. Although the piston would thus now be able to shift into its engaged position by itself, the high positive pressure built up in the hydrodynamic circuit—which had no effect previously while the piston was not moving—now goes suddenly into effect, exerting a strong axial force which accelerates the engaging movement of the piston, so that the piston travels at a very high velocity over the last part of its engaging stroke and thus enters into working connection with the axially adjacent, drive-side component of the clutch housing, such as, for example, a housing cover, in a very abrupt manner. As a result, the speed difference previously existing between the drive and the takeoff disappears within a very short time. In a vehicle traveling in push mode, this process is felt as an unpleasantly hard torque surge, which detracts from the comfort of the vehicle's passengers and also reduces the service life of the clutch device itself.